The production of semiconductor wafers is a multibillion dollar industry, which feeds an even larger manufacturing sector based on consumer electronics. This rapidly growing sector places an ever-increasing demand on semiconductor production. Thus, even small improvements in semiconductor manufacturing process control can have a large impact on the economic viability of companies, products, and entire industries.
A critical factor in all semiconductor production processes is improved control of production processing parameters, particularly temperature. The driving force for better process control is the need to reduce integrated circuit feature size. Unfortunately, reduced feature size translates directly into less tolerance for errors, and the need for finer control over the entire production process.
For example, there is a dramatic dependence on maintaining a specified process temperature to ensure uniform deposition of polysilicon on a semiconductor wafer. At a Silane partial pressure of 10 Pascals, the deposition rate changes by over 18 Angstroms per minute for a 1.degree. C. variance in processing temperature, which equates to about a 3% to about 20% change in the total deposition layer thickness each minute, over temperature. Other processes, such as Molecular Beam Epitaxy (MBE), chemical vapor deposition (CVD) and thermal annealing have even greater temperature dependence. All such processes stand to benefit from improved temperature control, as small variations in temperature can change the quality and functional properties of the wafers. This is even more important as wafer sizes increase, since, for example a 300-mm diameter wafer should be able to provide up to 2.5 times more chips than the 200-mm wafer.
Current methods for measuring temperature during semiconductor processing have several drawbacks. For example, thermocouples are fairly accurate, but are also invasive to the process. They cause Acool @ spots at the contact point and can only provide information at that point. For those processes where the gas environment composition is critical, thermocouples may also introduce contamination and oxidation problems.
Optical pyrometery provides a noncontact solution to this problem, but is hindered by changes that occur in the index of refraction, transmissivity of windows, and the emissivity of the wafer during the process. Stray radiation from wafer heating lamps also influences results, as does the inability to accurately know the emissivity for any particular wafer. Dual-wavelength pyrometer systems have been attempted as a way of obtaining emissivity-independence, but introduce other difficulties, such as requirements for distance calibration, and proportional emissivity throughout the measurement temperature range.
Laser ultrasonics (LU) to measure wafer temperature has been investigated for several years, but has not yet been demonstrated to accomplish industrial goals in a practical fashion. Ultrasound in the wafer is generated by relying on a basic thermoelastic mechanism in which the laser beam penetrates a short distance into the wafer before being completely absorbed. The light intensity must change very rapidly, generally in the form of a brief pulse. This raises the temperature of the penetrated volume of material before heat can escape, causing expansion of the surrounding medium and creating an acoustic pulse. Most of the stress applied to the wafer is in the radial directions on the surface plane, and thus LU is an efficient generator of surface waves, or guided waves. If the power of the laser is increased, the wafer surface begins to melt, and ultimately, the vaporization point is reached if even more power is applied.
The frequency spectrum of wafer-generated ultrasound is related to the temporal modulation of the laser. In principle, a laser beam could be modulated sinusoidally at the desired ultrasonic frequency, generating single tone ultrasound. In practice, a large average laser power would be needed to obtain significant ultrasound amplitudes, and the wafer under inspection would be heated appreciably. Instead, almost all laser-ultrasonics work has used pulsed lasers, which offer high peak intensities but low average powers. The ultrasound generated is intrinsically broadband. Pulses between 1 and 50 nsec long are used to obtain ultrasound peak amplitudes in a frequency range of about 0.5 and 25 MHZ. Even with pulses of this duration, the generated ultrasound ranges from DC to about 100 MHZ.
Lamb waves are guided acoustic waves that travel between two free boundaries. Their existence depends on the product of the acoustic frequencies and the thickness of the material. Thus, Lamb waves are well suited to investigate the behavior of plates and plate-like structures, such as wafers. The acoustic energy in Lamb waves may propagate in one or more modes (infinite modes are possible). However only the fundamental (zereoth order) symmetric and anti-symmetric modes can propagate for all frequency-thickness combinations. Furthermore, the fundamental anti-symmetric (A.sub.0) mode has much greater amplitude than the symmetric mode, and is thus more effectively measured and analyzed. Therefore, its use is preferred. The phase velocity of the Lamb wave depends on the combination of frequency, thickness, and the material properties of the plate. Variations of either thickness or material properties cause distinct phase velocity changes. The theory for deriving the dispersion relationships is illustrated in Rayleigh and Lamb Waves (Plenum, N.Y., 1967), by I. A. Viktorov, incorporated herein by reference in its entirety.
It has been determined that lasers can efficiently generate the lowest order anti-symmetric (A.sub.0) mode of the Lamb wave, which is sensitive to changes in temperature and to the wafer film coating thickness. In fact, using various techniques, experimental measurement accuracies of about .+-.0.15.degree. C. have been obtained using the changes in velocity of the A.sub.0 Lamb wave mode. The governing equation for the anti-symmetric Lamb wave mode is presented by a number of well-recognized sources. A basic form of the equation is given below: ##EQU1##
such that .omega. is the angular frequency, k.sub.a is the wave number of the A.sub.0 Lamb mode, c.sub.L is the longitudinal wave speed, c.sub.T is the transverse wave speed, and h is the half thickness. Variations of this equation can be derived for propagation in specific directions that coincide with the cubic crystal structure of the semiconductor materials.
Different optical detection schemes exist to measure temperature using ultrasound, including the fiber Fizeau interferometer, and the Aknife-edge @ detector. A fiber Fizeau interferometer is one of the simplest, most sensitive, ultrasonic interferometric detectors available. In this detector configuration, light reflected by the monitored surface interferes with light reflected at an intermediate point, typically at the exit surface of the illuminating fiber or lens. Thus, the amplitude of the detected light is modulated by changes in the distance between the optics and the wafer.
The Fizeau interferometer is in essence a single-pass Fabry-Perot interferometer wherein the cavity is formed by an air gap between the probe and the surface. Its main drawback is that this gap has to be maintained at an odd number of quarter optical wavelengths. Thus, environments with mechanical vibrations mandate some sort of position feedback to maintain this so called Aquadrature @ condition.
The Fizeau works well at room temperature, detecting laser-generated Lamb waves in wafers with good signal-to-noise ratio. However, when used in a furnace environment, the return signal is severely degraded. This can be attributed to changes in the cavity effective optical length due to air currents, and expansion of the wafer holder. Further, while the Fizeau enables ultrasonic inspections without contact, general optical methods are employed to couple ultrasound into the component, and to detect the corresponding (temperature-dependent) ultrasonic echoes. Thus stability issues continue to exist, as well as the expense involved in constructing a system with a minimum of expensive optical components.
The knife-edge detector is also capable of making basic temperature measurements, after a fashion. However, this instrument is based on the principle of specular reflection, which requires a highly polished and nearly optically-flat surface to ensure accurate results.
Other temperature measurement approaches have also been attempted in the processing of semiconductor wafers, using laser-generated ultrasound, but none has proved to provide stable, accurate, and inexpensive process control capability. These include interferometric-based materials analysis systems that employ shaped laser beams, and signal processing techniques to compensate for inherent instabilities of the diode lasers used in such systems. Matched-filter processing techniques are also employed to analyze Lamb L-waves generated within silicone wafer targets; however, such a system is cumbersome, requires a special optical bench to support it, and includes many expensive and critically-constructed optical components.
Therefore, what is needed is a laser-generated ultrasound process control system for semiconductor wafers which is inexpensive, rugged, and relatively insensitive to heat, stand-off distance, and air currents which may result from the process itself